In the case of a cold start and in the case of warm running of a spark-ignition engine, in some cases considerable quantities of fuel are input in an engine oil in the crank casing. During a further course of the heating up of the spark-ignition engine this fuel vaporizes out of the engine oil, for example at relatively high engine temperatures, and is discharged into an intake manifold of the spark-ignition engine via a crank casing venting system. A desired fuel/air mixture is fed to the cylinders of the spark-ignition engine through the intake manifold. The additional introduction of the vaporized fuel from the crank casing into the intake manifold causes the fuel/air mixture in the intake manifold to be enriched. This additional fuel in the intake manifold can in extreme cases, in particular when ethanol-containing fuels are used, bring about massive over-enrichment of the fuel/air mixture and therefore over-enrichment of the spark-ignition engine.
Over-enrichment is usually compensated by a Lambda controller. The Lambda controller is a sensor which measures the respective residual oxygen content in the combustion discharge gas of the spark-ignition engine, in order to be able to regulate on the basis thereof the ratio of the combustion air to fuel for the rest of the combustion, in such a way that, for example, neither an excess of fuel nor an excess of air occurs. If, for example, unburnt fuel in the combustion discharge gas is measured, the fuel/air mixture is set to be leaner in the intake section. The objective is to generate, as a function of the fuel used, a lambda value of λ≈1 in the fuel discharge gas, without severe deviations occurring therefrom.
Correspondingly measured lambda deviations are used, for example, for diagnostic purposes (what is referred to as fuel system diagnosis FSD) and for detection of the proportion of ethanol in the fuel. If a lambda deviation occurs which arises as a result of the outgased fuel which is fed into the intake section, incorrect error detection of the diagnosis or an incorrectly measured ethanol proportion value can be caused. It is therefore important to detect whether the lambda deviation has been caused by fuel discharge from the engine oil or, for example, by a defect in the spark-ignition engine itself.
Previously, in order to detect possible fuel discharge from an engine oil, what are referred to as fuel input/discharge models in engine control were calculated and compared with different operating conditions of the engine. Since the fuel input/discharge depends very greatly, inter alia, on the fuel used (fuel quality, ethanol proportion), such fuel input/discharge models can only make a statement as to whether fuel discharge from the engine oil is theoretically possible. If the spark-ignition engine is running under operating conditions in which, according to the calculated oil input/discharge model, it is possible for fuel to be discharged, the diagnoses described above, ethanol proportion detection processes or lambda adaptations are disabled in order to prevent any incorrect measurements being obtained. The actual quantity of fuel discharge can as a result be estimated only coarsely. Such detection of the fuel discharge by means of fuel input/discharge models can also be achieved only at very high cost.
Alternatively, what is referred to as a HC sensor (hydrocarbon sensor) can be installed in the crank casing venting means. The HC sensor can measure the proportion of the hydrocarbons from the fluid vented from the crank casing. However, such HC sensors are expensive and therefore tend not to be very suitable for use in series fabrication.